Magnetic stimulation is a noninvasive tool for the study of the human brain and peripheral nerves that is being investigated as a potential therapeutic agent in psychiatry and neurology. When applied to the brain, this technique is commonly referred to as Transcranial Magnetic Stimulation (TMS). However, the term “TMS” is often used to refer to magnetic stimulation of other body organs as well. Therefore, the term TMS will be used hereinafter to refer to magnetic stimulation of the brain or other body organs.
In TMS, a pulsed current sent through a coil produces a magnetic field that induces an electric field in the brain, which can affect neuronal activity. A single TMS pulse can activate a targeted brain circuit. For example, a TMS pulse delivered to the motor cortex can result in a twitch of the associated muscles in the body. Further, a single TMS pulse can also disrupt neural activity. For example, a TMS pulse delivered to the occipital cortex can mask the perception of a visual stimulus. This allows researchers to probe brain circuits on a millisecond time scale.
A train of TMS pulses, referred to as repetitive TMS (rTMS), can produce excitatory or inhibitory effects which last beyond the stimulation interval. Repetitive TMS provides a means to study higher cognitive functions, and it could potentially be used as a therapeutic intervention in psychiatry and neurology.
The neural response to TMS is sensitive to the parameters of the stimulating TMS pulse. The pulse width (PW), shape (e.g., sinusoidal vs. rectangular), and the relative amplitude of the positive and negative phases (degree of bidirectionality) of the induced electric field affect the physiological response to TMS, the power efficiency of the stimulator, and the heating of the stimulating coil. Existing TMS systems are capable of inducing only damped cosine electric field pulse shapes, with a limited set of discrete choices of pulse width and degree of bidirectionality. Further, in existing TMS systems, monophasic magnetic field pulse shapes are associated with very low power efficiency of the stimulator in rTMS applications.